This patent specification is in the field of radiography and pertains more specifically to x-ray imaging using a digital flat panel detector.
Flat panel x-ray imaging devices that generate electrical signals related to local x-ray exposure have been developed in recent years. An example is discussed in U.S. Pat. No. 5,319,206, the contents of which are hereby incorporated by reference, and a current version is commercially available from the assignee of this patent specification. These panels offer good spatial resolution and dynamic range properties and can replace x-ray film in a variety of radiographic procedures, such as, without limitation, chest x-ray imaging. However, when the exposure level is much lower than used for procedures such as chest x-rays, for example at the low exposures used for procedures such as real-time fluoroscopy, electronic noise and other adverse characteristics can detract from optimal performance of these flat panel devices. In addition, in certain applications other that real-time fluoroscopy, such as in procedures where it is particularly important to reduce patient x-ray dosage, it may also be desirable to image at lower than conventional x-ray exposure levels.
Real-time fluoroscopic radiation exposures can be 10 to 1000 times lower than typical radiographic exposures such as chest x-rays. Image intensifiers are commonly used in these low-exposure applications, and because of their high gain they can generate an acceptable signal-to-noise performance even at very low x-ray exposure levels. Image intensifiers unfortunately have undesirable properties, such as spatial distortions, poor contrast, and are heavy and large.
Because of other advantages of flat-panel detectors used as imagers, there exists a need for a flat-panel device that can operate with high performance at the very lowest x-ray exposure levels. There also exists a need for a flat panel device to operate at these low x-ray exposure (dose) levels that is also capable of generating many images per second for real-time imaging. There also exists a need for a flat panel device that operates at these low dose levels that does not have spatial distortions and other imaging non-uniformities and that is small and lightweight as compared with an image intensifier of a comparable image plane size.
The flat panel x-ray imaging device such as disclosed in U.S. Pat. No. 5,319,206 comprises an image charge generation layer such as selenium, and a flat panel thin film transistor layer with a multiplicity of pixels and signal control and output lines. A pixel includes a charge collector, image charge storage capacitor, and field effect transistor. (The term pixel is used here to denote a portion of the flat panel detector that can generate an electrical signal for a positionally corresponding display pixel on a screen displaying an x-ray image.) The x-ray detection lower limit of such a device depends to a large extent on the charge signal generated from the incoming x-rays and the level of electronic noise in the readout circuit.
Such a flat panel device can be enhanced to improve its ability to image at low x-ray exposure through enhancing the charge signal produced from the incoming x-ray and therefore increasing the sensitivity of image formation and producing a radiation image from a very low level of radiation. This can be done through the use of a gain or charge multiplication layer in addition to known layers of a flat panel detector, as described in greater detail below.